Method for interior zone melting of a crystalline rod



June 28, 1966 J. A. REDMOND ETAL 3,258,314

METHOD FOR INTERIOR ZONE MELTING OF A CRYSTALLINE ROD Filed April 12,1963 l4 13 A0. LOWER HIGH POWER FREQUENCY FREQUENCY SUPPLY POWER 25POWER SUPPLY SUPPLY IO I2 2 55% E1 K 1 I K" *3 C- 5 1 "H33 COOLING "I I6MEDIUM I: SOURCE E@ CIRCULATING MEANS Fig-L Fi g A.C. 7 POWER SUPPLY 252 25 Fig.3. 3 Flg. 4.

R 5 Fig.5. WITNESSES INVENTORS M J -W John A-Redmond Eugene JublonskiBYE 3" k United States Patent 3,258,314 METHOD FOR INTERIOR ZONE MELTING OF A CRYSTALLINE ROD John A. Redmond, Ellicott City, and EugeneJablonslri,

Baltimore, Md., assignors to Westinghouse Electric Corporation,Pittsburgh, Pa., a corporation of Pennsylvania Filed Apr. 12, 1963, Ser.No. 272,676 5 Claims. (Cl. 23 -301) The present invention relates to amethod and apparatus for zone melting, and more particularly to a methodand apparatus for crucible-free floating-zone melting of elongatedmaterial such as semiconductor material, refractory metal, etc. inrod-shaped form, according to which a longitudinal molten zone is causedto move axially along the material while it is held in a verticalattitude.

Various techniques are employed to obtain the molten state of thelongitudinal zone of the rod material, such, for example, as by electronbombardment heating and induction heating. The liquefied material of athroughmelted or floating molten zone between the adjacent upper andlower solid portions of the rod is believed to be retained between suchportions primarily by surface tension, although in the case of heatingby induction a levitating influence. from the magnetic field of the coilcan be obtained from a specially-designed induction coil to contributeto confinement of such molten zone. In any event, the practice has beento subject a selected lengthwise zone of the rod material to a singlesource of heat, such as a cathode coil for electron bombardment heatingor an induction coil for induction heating, for a sufficient period oftime to through-melt the rod. In accord with the usual practice, designand operating parameters are chosen to produce the molten zone in suchprevious fashion while causing such zone to scan the rod verticallyupward by effecting relative axialwise movement between the rod and anencircling induction heating coil or an encircling electron bombardmentheating coil This practice is entirely satisfactory in many respects,but has resulted in limitation of the maximum diameter of rod which maybe floating zone-melted without spill-over from the uniformlythrough-melted zone. For silicon rod, for example, this maximum diameterwith induction heating and levitating techniques appears to be 1%inches, and for tungsten rod, for another example, it appears to beconsiderably less.

In view of the foregoing remarks, it becomes a prime object of thepresent invention to provide improvements in the aforedescribed methodand apparatus whereby larger diameter rod-shaped material may befloating zone melted.

In accord with such object, general features of a longitudinal zone ofthe invention includes the melting of the rod material internally by oneheat source, while an outer solidified cup-shaped or bowl-shaped shellis maintained in the rod material in containment of a considerableportion of the inner molten material, and the melting of the rodexternally by another heat source to give a complete through-meltingonly at the upper edge of the shell and at the top of the molten zonefor a sufliciently narrow through-melted axial distance to maintain themolten material confined radialwise between the bottom of the uppersolidified portion of the rod and the upper unmelted rim of thesolidified shell. Such internal and external melting of the rod shouldbe effected simultaneously By virtue of this novel technique, the axiallength of the floating or through-melted part of the molten zone may bekept sufiiciently narrow to assure containment of a larger diameter zonethan heretofore possible and therefore a large diameter rod can befloating zone melted than heretofore. It will be understood that the rodmaterial is scanned by the molten zone as thus established, thisscanning which per se forms no part of this invention, can be effectedby any suitable means which obtains relative axialwise movement betweenthe rod material and'the heat sources. Rotation of the rod also isdesirable and can be obtained in well known manner.

Other features, objects, and advantages of the invention will becomeapparent from the following detailed description taken in connectionwith the accompanying drawings in which:

FIGURE 1 illustrates in elevation, partly in outline and partly insection, the internal heating effect of a liquidcooled induction heatingcoil on a rod-shaped material when functioning in accord with a featureof the invention;

FIGURE 2 illustrates the effect of melting the rod material internallywith the liquid-cooled coil of FIGURE 1 at one power supply frequency,and melting through the outer periphery of the rod material by a secondinduction coil at a higher power supply frequency;

FIGURE 3 illustrates an alternate technique substituting electronbombardment heating in lieu of the externalheat induction coil of FIGURE2; and

FIGURE 4 illustrates another alternate arrangement for obtaining theexternal heating which employs a reflected radiation technique; and

FIGURE 5 is a cross-sectional view of a turn of a combined liquid-cooledcoil and cooling gas coil.

-Referring now to the exemplification in FIGURE 1, in accord with afeature of the invention as discussed generally hereinbefore, theinternal heating of the vertically-extending rod materialis'accomplished by an encircling multi-turn water-cooled, hollow-tubing,induction coil 2 which is energized from an alternating current powersupply- 3 by alternating current at a frequency and power levelsufiicient to heat a longitudinal section or segment of the rod andcause an inner region 4 of the zone to melt while an outer encirclingsolid wall 6 remains unmelted by such coil by virtue of dissipation ofheat therefrom, as by radiation to the cooled coil 2 and its environs. Afrequency of ten kilocycles per second, for example, at a power level of40,000 watts with two-turn A x A" square section hollow copper coilcooled to F. will'internally melt a .675 diameter tungsten rod with a.040 inch approximately thick outer wall 6. Considerably larger diameterrods of tungsten or other refractory metal, similarly can be internallymelted with suitable choice of frequency, power level, and coiltemperature.

The forming of the outer wall 6 in encirclement of the inner region 4 ofmolten zone of the rod 1 is'dependent upon dissipation of heat from thesurface of the 'rod, as

by radiation to its environment and this rate is a function of theabsolute temperatures to the fourth power between rod 1 and suchenvironment. It will be seen that the higher the melting point of therod material, the greater will be the capability for heat to radiatefrom its outer surface and facilitate realization of the desired surfacecooling effect by radiation alone. For example, tungsten which has amelting point of about 6170 F. (3410 C.) readily radiates sufficientheat to a coil and housing even at 100 F. to obtain the solid-shellmolten-- interior encasement effect, which would be true also of otherrefractory metals such as molybdenum. Material such as silicon, whichhas a melting point of 2605 F. to appreciate the same result byradiation cooling effect alone may not be attainable even with cryogeniccooling of the coil 2. In this case, surface cooling of the silicon rodwithin such coil may be aided by directing a flow of gas over the outersurface of the rod material as by way of the interior of the coil 2 andorifices '7 at the inner periphery of the coil turns, as in FIGURE 2,'where such cooling gas may also serve to cool the coil, or via a separate gas manifold 8 formed separately or integrally with the coil 2which may be separated by a partition 9 from an interior portion 10 ofthe coil through which cooling water is circulated in a well-knownmanner. Exemplified structural details of such a coil turn are shown incrosssection in FIGURE 5.

To effect the through-melting of the rod wall 6 at the top of the innerregion 4 of the molten zone within a narrow axialwise portion 10 inaccord with the invention as aforediscussed herein, different techniquesmay be employed. For example, as in FIGURE 2 by employment of asingle-turn water-cooled induction coil mounted above the coil 2 andenergized from a power supply 13 of higher frequency than that supply 14from which the coil 2 is energized. The lower frequency energization ofcoil 2 performs the deeply penetrating heating effect for the interiormelting, and the higher frequency energization of coil 12 concentratesits heating effect in the outer portion of the rod to melt through thewall 6, such latter concentration of outer surface heating acting toovercome the heat dissipation effect of radiation from the rod. Asexemplified in FIGURE 3, in lieu of coil 12, it can be accomplished byelectron bombardment heating by electron emission from aninwardly-directed filament turn 16 connected to one side of a highvoltage D.C.'power supply 17 which is connected at its opposite side tothe rod 1. Or, as exemplified in FIGURE 4, such surface-concentratedheating can result from re-radiation of heat from the rod back to theselected surface region by means of a reflector ring 20'0f suitablefocusing configuration. In all cases, it will be understood thatrelative movement between the rod 1 to be floating-zone melted, as forconversion from polycrystalline to monocrystalline form, and the coil 2and auxiliary final-melt heat source 12, 16 or 20 is required to effectprogressive movement of the molten zone along the length of the rod, asis well-known. The rod 1 also is rotated during such progressive zonemelting to obtain a uniform heating effect from the encircling heatcreating sources and to visually confirm the creation of a thru-melt.Such movements may be effected in a manner as described and shown in ourcopending US. patent application, Serial No. 219,957, filed August 28,1962, titled Apparatus for Zone Heating, or in other suitable ways. Itis assumed herein that the workpiece rod 1 is moved axially relative tothe heating coil, as indicated by arrows in the drawings. However,insofar as the present invention is concerned, relative axialwisemovement may be obtained by movement of the heating coil as has beentaught heretofore by others.

Although not shown in the drawings, the zone melting method of thepresent invention will be carrried out in an enclosure which may be anevacuable one, a vacuum being necessary in the technique exemplifiedstructurally in FIG- URE 3 employing electron bombardment, and whichenclosure alternatively may contain various gases such as argon, helium,hydrogen, etc, in which case the gas for cooling the rod 1 as in FIG-URE2 would be recoverable for recirculation following suitable cooling andtreatment for removal of contaminants.

While there have been shown and described what are at present consideredto be the preferred embodiments, uses, and advantages of the invention,modifications thereto and other uses and advantages may readily occur tothose skilled in the art. It is not desired, therefore, that theinvention necessarily be limited to the specific arrangements shown anddescribed, and it is intended to cover in the appended claims all suchmodifications and uses as fall within the true spirit and scope of suchinvention.

We claim as our invention:

1. In a method of floating-zone melting a rod-shaped materialsusceptible to floating-zone melting, in which the rod is verticallydisposed and a molten zone is created in the rod and caused to movelengthwise therealong, the improvement for creating such molten zonecomprising the steps of heating a longitudinal segment of the rod toobtain interior melting thereof while heat is dissipated from its outersurface to preserve an elongated solid outer shell in encirclement ofthe molten interior portion of the zone, and heating an axialwise narrowregion of said shell at the upper portion of the molten zone to givethrough- -melting of the rod exclusively at such narrow region.

2. In a method of floating-zone melting a rod of refractory metalmaterial susceptible to floating-zone melting, in which the rod isvertically disposed and a molten zone is created in the rod and causedto move lengthwise therealong, the steps for creating such molten zone,inductively heating a longitudinal segment ofthe rod at a frequency andpower level which causes melting of the interior thereof whiledissipation of heat by radiation from the exterior preserves a solidouter wall which encircles the molten interior, such molten interior.being of greater overall length than can be retained between adjacentupper and lower solid rod portions as a floating zone through-melted forsubstantially the entirety of such length, and additionally heating anaxialwise region of said outer wall at the upper portion of the moltenzone to giVe through-melting of the rod exclusively in a sulficientlynarrow region to permit such retention of the molten material.

3. In a method of floating-zone melting of a verticallyextendingrod-shaped material susceptible to floating-zone melting, the steps ofcreating the molten zone by melting a longitudinal segment of theinterior of the rod while its outer surface remains solid except for anarrow through-melted region at the top of lesser axial length than theaxial length of the overall molten zone, such molten interior being ofgreater overall length than can be retained between adjacent upper andlower solid rod portions as a floating zone through-melted forsubstantially the entirety of such lentgh, and causing the thus-shapedmolten zone to be moved axially along the rod.

4. In a method of floating-zone melting of a rod-shaped materialsusceptible to floating-zone melting, in which the rod is verticallydisposed and a molten zone is created in the rod and caused to movelengthwise therealong, the creating of such molten zone by the heatingof a longitudinal segment of the rod while selectively cooling areas ofits outer surface in said segment such that within the segment itsinterior becomes molten and its exterior remains solid except for anarrow through-melted region at the top of the molten-zone, such molteninterior being of greater overall length than can be retained betweenadjacent upper and lower solid rod portions as a floating zonethrough-melted for substantially the entirety of such length.

5. In a method of floating-zone melting a floating-zonemeltable materialin rod shape, in which the rod is vertically disposed and a molten zoneis created in the rod and caused to move lengthwise therealong, thesteps for forming the molten zone by penetratedly heating a longitudinalsegment of the rod while providing for dissipation of heat from itsouter surface, thereby creating an elongated interior of molten materialencased in solid material, and surface heating of a narrow portion atthe top of such molten interior to melt such encasing solid material andobtain through-melting of the molten zone exclusively at such narrowportion, such molten interior being of greater overall length than canbe retained between adjacent upper and lower solid rod portions as a.floating zone through-melted for substantially the entirety of suchlength.

References Cited by the Examiner UNITED STATES PATENTS 3,023,091 2/1962Smith 23301 3,121,619 2/1964 Scholte 23--301 OTHER REFERENCES Hannay:Semi-conductors, Reinhold Publishing Company, 1959, pages 122-123.

Wroughton 2119-45 X Pfann: Zone Melting, John Wiley and Sons, page 77 toHull et al. 23-301 X 79, 1958.

Cornelison 23-301 Matare 23273 X 9 NORMAN YUDKOFF, Primary Examiner.

Gwllssen 2340 G. HINES, A. J. ADAMCIK, Assistant Examiners. Emeis23--301

1. IN A METHOD OF FLOATING-ZONE MELTING A ROD-SHAPED MATERIALSUSCEPTIBLE TO FLOATING-ZONE MELTING, IN WHICH THE ROD IS VERTICALLYDISPOSED AND A MOLTEN ZONE IS CREATED IN THE ROD AND CAUSED TO MOVELENGTHWISE THEREALONG, THE IMPROVEMENT FOR CREATING SUCH MOLTEN ZONECOMPRISING THE STEPS OF HEATING A LONGITUDINAL SEGMENT OF THE ROD TOOBTAIN INTERIOR MELTING THEREOF WHILE HEAT IS DISSIPATED FROM ITS OUTERSURFACE TO PRESERVE AN ELONGATED SOLID OUTER SHELL IN ENCIRCLEMENT OFTHE MOLTEN INTERIOR PORTION OF THE ZONE, AND HEATING AN AXIALWISE NARROWREGION OF SAID SHELL AT THE UPPER PORTION OF THE MOLTEN ZONE TO GIVETHROUGHMELTING OF THE ROD EXCLUSIVELY AT SUCH NARROW REGION.